The solar power market has continuously grown in popularity and the ability to create high-efficiency solar cells is a key strategy to meet the growing world energy needs. Today's photovoltaic systems are predominantly based on the use of crystalline silicon, thin-film and concentrator photovoltaic technologies.
Thin-film technologies have lower efficiencies than crystalline silicon cells but permit direct application to a surface that can be plastic. Thin-film technology reduces end product costs because it allows for smaller amounts of semiconductor material to be used, can be manufactured by a continuous process, and results in a product that is less likely to be damaged during transportation.
Thus, a promising low cost alternative to silicon solar cells can be found in organic photovoltaic devices (OPVs), if their power conversion efficiency can be increased comparable to common devices (Landi, B. J.; Raffaelle, R. P.; Castro, S. L.; Bailey, S. G., (2005). “Single-wall carbon nanotube-polymer solar cells”. Progress in Photovoltaics: Research and Applications 13 (2): 165-172) and if moderate power conversion efficiencies can be achieved at low costs.
Organic (polymer-based) solar cells are flexible, and according to the current state of development, their production costs are about a third of the price of silicon cells. They are disposable and can be designed on a molecular level. Current research is focusing on the improvement in efficiency and development of high-quality protective coatings to minimize the environmental effects.
Such organic photovoltaic devices (OPVs) may be fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Since polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. This means, OPVs based on conjugated polymers can be fabricated by highly scaleable, high speed coating and printing processes enabling rapid mass-production. OPVs' low cost and manufacturing ease make them attractive even if their efficiencies are lower than that of existing technologies. Subsequently there is a large amount of research being dedicated throughout industry and academia towards developing new OPVs.
Nanotechnology is currently enabling the production of organic photovoltaics (OPVs) to overcome the disadvantages associated with traditional silicon based photovoltaics. Organic photovoltaics are composed of layers of semiconducting organic materials (polymers or oligomers) that absorb photons from the solar spectrum. In OPVs, solar radiation promotes the photoactive semiconducting organic materials in the photoactive layer to an excited state. This excited state is referred to as an exciton and is a loosely bounded electron-hole pairing.
This was made possible by the discovery of photoinduced electron transfer from the excited state of a conjugated polymer (as the donor) onto fullerene (as the acceptor). Fullerene provides higher electron separation and collection efficiency compared to previously known electron acceptors.
Photovoltaic cells based on polymer/fullerene C60 planar heterojunctions have been reported earlier. Blending a conjugated polymer and C60 (or its functionalized derivatives) results in moderate charge separation and collection efficiencies due to the formation of bulk donor-acceptor (D-A) heterojunctions.
In this context carbon nanotubes (CNTs) have also attracted great interest with their nano-scale cylindrical structure. Carbon nanotubes (CNT) layers seem very promising and applications of CNTs in OPVs are of much interest. Depending on the varied chiralities (the arrangement of the carbon honeycomb with respect to its axis), CNTs can be semiconducting or metallic with nearly ballistic conduction. CNTs, especially single wall carbon nanotubes (SWCNTs), are known as excellent electron transporters. SWCNTs have in fact already been employed as electrodes and blended with conjugated polymers to form bulk heterojunctions in the active layers. Kymakis et al. first reported a photovoltaic device based on the blend of SWCNTs and the conjugated polymer poly(3-octylthiophene) (P3OT) [E. Kymakis; G. A. J. Amaratunga, Solar Energy Materials and Solar cells 80, 465-472 (2003), “Photovoltaic Cells Based on Dye-Sensitization of Singel-Wall Carbon Nanotubes in a Polymer Matrix”]. Adding SWCNTs to the P3OT matrix improved the photocurrent by more than two orders of magnitude. In a another work, Pradhan et al. blended functionalized multi-walled carbon nanotubes (MWCNTs) into a poly(3-hexyl-thiophene) polymer (P3HT) to provide extra dissociation sites and assist in charge transport in a P3HT-MWCNT/C60 double-layered device [B. Pradhan, Ksetyowati, H. Liu, D. H. Waldeck, J. Chen, Nano Letters 8 (4), 1142-1146 (2008)].
However, nanotubes distributed within a polymer matrix are less efficient in separating photogenerated carriers than spherical C60 molecules that have a larger surface to volume ratio and it is difficult to disperse CNTs in a photoactive matrix.
CNT layers have a transparency in the visible and IR spectrum and because of their electrical properties CNT coatings and circuits are becoming one of the latest alternatives to traditional conductive materials [e.g. indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide or cadmium sulfide] [R. Saito, G Dresselhaus, M. S. Dresselhaus, “Physical Properties of Carbon Nanotubes”, Imperial College Press, London U.K. 1998]. Carbon nanotubes are an allotrope of carbon that is found in both a single-walled carbon nanotube and multi walled carbon nanotube variety. Carbon nanotubes are known to exhibit extraordinary strength, heat conductance, and electrical properties.
Known CNT networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency: They are easier and cheaper than ITO to deposit on glass and plastic surfaces, since they can be formed into a solution, compared with ITO which has to be sputtered onto a surface in a vacuum. This is why it is desirable to apply CNT layers and to replace ITO layers in photovoltaic devices.
Combination of CNTs with electron donors signifies an important concept to harvest solar energy and convert it into electricity. Like C60, CNTs have been introduced into the same conjugated polymers to produce organic photovoltaic devices [E. Kymakis, G. A. J. Aramatunga, Rev. Adv. Mater. Sci. 10, 300-305 (2005)].
Most importantly, CNT-based hetero junctions are of particular interest because of their unique geometry as well as excellent electronic, thermal and mechanical properties. Free electron/hole pairs excited by photons can be either separated by an externally applied voltage, by internal fields at the Schottky barriers, at p-n junctions or at defects. A photocurrent or a photo voltage can be generated.
The photo current in CNT junctions shows band-to-band transitions and photon-assisted tunneling with multiple sharp peaks in the infrared, visible and ultraviolet. Besides individual CNT, CNT macro-bundles and films also produce a photocurrent. Windows/back electrode made from CNTs is yet another important application in solar cells. Thin, transparent layers comprising bulk metallic CNTs have been proposed for providing lateral (in-plane) electrical conductivity for collecting current from the front surface of thin-film solar cells.
At present further developments of OVPs comprising CNTs is intensified in order to make them ready for the market.
JP 2005327965 (A) discloses a photovoltaic device using carbon nanotubes, especially multilayered carbon nanotubes, wherein carbon nanotubes are dispersed in a medium, which is laminated on a conductive substance. The latter may be an aluminium or a copper foil. The surface of the CNT comprising layer is brought into contact with a collector or an electrode. A transparent protective layer, which may consist of silicone rubber or plastics, is further laminated on the carbon nanotubes comprising layer or on the conductor obtained by dispersing the carbon nanotubes in a medium.
Different methods of forming layers comprising carbon nanotubes are known. For example CNTs are deposited from the gaseous carbon feedstocks in the presence of unsupported catalysts (see U.S. Pat. No. 6,221,330) or carbon vapour is produced by electric arc heating of solid carbon and contacting the carbon vapour with cobalt catalyst (U.S. Pat. No. 5,424,054).
US 2002/0025374 A1 discloses a selective growth method on a substrate to form patterned carbon nanotubes. The nanotubes are grown directly on a surface at high temperatures >500° C. This limits this technology to substrates withstanding high temperatures.
In U.S. Pat. No. 6,835,591 a conductive article is disclosed, which includes an aggregate of nanotube segments in which the nanotube segments contact other nanotube segments to define a plurality of conductive pathways along the article. The articles so formed may be disposed on substrates, and may form an electrical network of nanotubes within the article itself. Conductive articles may be made on a substrate by forming a nanotube fabric on the substrate, and defining a pattern within the fabric in which the pattern corresponds to the conductive article. The nanotube fabric may be formed by depositing a solution of suspended nanotubes on the substrate. The deposited solution may be spun to create a spin-coating of the solution. The solution may be deposited by dipping the substrate into the solution. The nanotube fabric is formed by spraying an aerosol having nanotubes onto a surface of the substrate.
Again in WO2008/051205 A2 CNT layers are formed in a two step method. First a dilute water solution of CNTs is sprayed on a substrate. Water is evaporated leaving only the consolidated carbon nanotubes on the surface. Then a resin is applied on the CNTs and penetrates into the network of the consolidated CNTs. The prepared layer shows a low electrical resistance as well as a high light transmittance. WO2008/051205 A2 also discloses the preparation of a photovoltaic cell, whereby an active layer is formed between a first and a second electrode. The first and the second electrode are transparent conductive coatings containing CNTs. The comprising CNTs may be single walled carbon nanotubes (SWNT) or multi-walled carbon nanotubes (MWNT). The active layer comprises an electron acceptor material and an electron donor material. The acceptor properties as well as the donor properties can be achieved by doping functionalizations or by placing a thin (2-6 nm) alkali fluoride layer between the first electrode and the active layer and between the active layer and the second electrode. The acceptor may be a fullerene component. The electron donor may comprise a conjugated polymer, like poly(3,4-ethylene dioxythiophene, or other polythiophene derivatives, polyaniline or other electron donor polymers. The electrodes may be mesh electrodes, which guarantee flexibility, made of metals, like palladium, platinum, titanium, stainless steel, and alloys thereof.
In all these cases carbon-based films, including but not limited to CNTs, single walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNT) or fullerenes (C60) may exhibit active components in transparent layers suitable for transforming solar energy into electricity. However, when CNT material is used to form a part of photovoltaic devices, the comprising CNT material often has a rough surface topography, with pronounced thickness variations, such as numerous peaks and valleys. The rough surface topography can cause difficulties. For example, the rough surface topography of CNT materials can make CNT materials difficult to be etched without undesired etching of the underlying substrate, or without increasing fabrication costs and associated with their use in integrated circuits.